Frequency synthesizers have been used to provide high-frequency signals within, for example, various types of communications equipment and measurement instrumentation. As is well known, at microwave frequencies and above the phase noise generated by reference oscillators included within such synthesizers can significantly degrade the spectral purity of the high-frequency output signal. Phase noise, i.e., frequency jitter, corresponds to the noise power generated by the synthesizer at frequencies other than the desired output frequency.
Oscillator circuits which include low phase noise (YIG) oscillators are often incorporated in microwave frequency synthesizers. The desirable phase noise characteristics of YIG tuned oscillators arise as a consequence of incorporation therein of YIG resonators, i.e., tuned oscillators, having high quality factors (Q). In order to further improve the phase noise performance of a particular YIG tuned oscillator, it may be phase-locked to a stable reference source as shown in FIG. 1.
Referring to FIG. 1, there is shown a block diagram of a conventional YIG tuned oscillator circuit 10 designed to phase lock a YIG tuned oscillator 14 to a stable reference oscillator 18. The output frequency of the reference oscillator 18 may be made to match the frequency to which the tuned oscillator 10 is tuned by, for example, translating the frequency of the stable reference source using a mixer (not shown). A portion of the RF output signal generated by the YIG tuned oscillator 14 is provided to a phase detector 26 by an RF coupler 28. The oscillation frequency of the YIG tuned oscillator 14 is controlled by externally adjusting the DC voltage applied to the coarse tuning input of the YIG tuned oscillator 14, and is further stabilized in accordance with a fine-tuning signal derived from a loop filter 32. As is indicated by FIG. 1, the loop filter 32 is connected between the output of the phase detector 26 and a fine-tune port of the YIG tuned oscillator 14.
FIG. 2 graphically represents the phase noise, as measured in decibels relative to the carder signal (dBc), of the reference oscillator 18 and of the YIG tuned oscillator 14. In the representation of FIG. 2 phase noise is plotted as a function of frequency offset from the carrier frequency to which the YIG tuned oscillator 10 is tuned. As is apparent upon inspection of FIG. 2, the phase noise induced within the tuned oscillator 10 by the reference oscillator 18 (dotted line) is less than the phase noise of the free-running YIG tuned oscillator 14 (dashed line) for frequencies up to a crossover frequency of about 10.sup.7 radians/second (.apprxeq.1.59 MHz) relative to the carrier frequency. By selecting the low-pass cutoff frequency of the loop filter 22 to be approximately equal to the crossover frequency, the overall phase noise of the tuned oscillator 10 (solid line) is made to be dominated by the reference 18 at frequencies less than the crossover frequency. At frequencies in excess of the crossover frequency, i.e., at those frequencies outside of the bandwidth of the phase-locked loop, the overall phase noise is determined by the YIG tuned oscillator 14.
As the phase noise performance of reference oscillators has continued to improve, the associated crossover frequencies at which the phase noise engendered thereby becomes dominant has correspondingly increased. In order to exploit such enhanced oscillator phase noise characteristics it is necessary that the phase-locked loop bandwidth be capable of being extended to such higher crossover frequencies. Unfortunately, signal loss induced by the magnetic core material of inductive coils used in tuning YIG resonators has tended to limit maximum loop bandwidth. Even when "fine tune" coils are utilized in resonator tuning it has proven difficult to attain loop bandwidths in excess of several hundred kHz, since such coils tend to cause phase lock loop instability by inducing excessive phase shift. Methods of resonator tuning not relying upon inductive coils (e.g., those using varactor diodes), have tended to degrade phase noise performance by inducing non-linear tuning characteristics or by lowering the oscillator Q.